Automatic volume control circuits



April 29, 1941.

V1:. E'. FOSTER E-rAL 2,239,844

AUTOMATIC VOLUME CONTROL CIRCUITS med oct. 4, 1938 AMpuF/ER MPL. Topp/0p7055s Lrl 1 704 22 3 l ff 4 4 i "uw" la awww( 1 @1 m A- E I; z: E 7 :t TE, r Q Tf f o I A YMW l Patented Apr. 29, 1941 UNITED STATES PATENTortica y 2,239,844 v AUTOMATIC' voLnME conraoi. omcnrrs Dudley E.Foster, South 0range,` N. J., land Garrard Mountjoy, Manhasset, N. Y.,assignor to Radio 'Corporation of America, a corporation of Delawaremessage october 4, 193e, serial No. 233,164

i claims. (ci. 25o-2t) provement involving the feeding back to thedetector input of signal energy in degenerative phase, and thesimultaneous use of the direct current voltage component of the detectedsignal energy for automatic volume control (AVC) action.

Another important object of the invention is to provide in a receivingsystem a detector having a normally non-linear detection characteristic,and detected signal energy being fed back in degenerative phase to atleast one network prior to the detector thereby to render saidcharacteristic substantially linear; the direct current voltagecomponent of the detected energy being utilized to maintain the carrieramplitude at the detector input circuit substantially uniform.

Another object of our invention is to improve radio receivers of thetype normally provided with -a non-linear detector, detected energybeing fed back to a preceding amplifier in degenerative phase therebyyto improve the linearity of the detector, and the direct current voltagecomponent of the detected energy being utilized to vary the amplier gainin a sense such as to maintain the carrier amplitude at the detectorinput substantially uniform.

Another object `of our invention is to provide a detector of the biasedtype wherein audio degenerative feed-back is .employed between theoutput and input circuits, and the detector characteristic is therebylinearized; the direct current voltage component in the detector outputcircuit being employed for varying automatically the gain of at leastone ampliiier preceding the detector.

Still other objects of our invention are generally to improve detectionsystems, and more especially to provide detection systems utilizingdegeneration and which systems are not only reliable and e'icient inoperation, but are economically manufactured and assembled in radioreceivers.

The novel features which we believe -to be characteristic of ourinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization and method ofloperation will best be 4understood byv Y ceiver.

reference to the following description taken in connection with thedrawing in which we have indicated diagrammatically several circuitorganizations, whereby our invention may be carried into eiiect..

In the drawing:

Fig.AV 1 Aschematically shows a circuit diagram embodying one form ofthe invention,`

Fig. 2 illustrates a modied form of the invention.y

Referring now` to Vthe accompanying drawing, wherein like referencecharacters in the different iigures indicate similar circuit elements,there is ,shown in Fig. 1 those networks of a radio Vreceiver which areessential to a proper understanding oi the present invention. It will beunderstood Aby those skilled in the art that the networks feeding theamplier tube I may be those commonly employed in any desired type ofreceiving system, such as a superheterodyne re- Let it be assumed thattube l is embodied inthe intermediate frequency (I. F.) ampliernetworkofthe receiver.` Hence, the input electrodes of tube I arecoupled to the tuned input circuit 2, and the latter is xedly resonatedto the operating I. F. Of course, the networks preceding the inputcircuit 2 will comprise the usual signal collector followed by avconverter stage which feeds once or more I. F. ampliiers.

The plate circuit of amplifier l includes the output circuit 3 which isfixedly tuned to the I. F. value, and circuit 3 is reactively coupled toinput circuit 4 of the detector tube 5; it being understood that circuit4 is also tuned tothe I. F.

The detector tube 5 is of the biased type; that is to say, it functionsas a plate rectification type of detector. The cathode 6 of tube 5 isconnected to an intermediate point on the Voltage supplyv bleederresistor The grounded side of resistor l is the positive terminal of thepotential source; plate 8 is connected to said terminal through a pathwhich includes load resistor 9 and the choke coil IU. The

input grid Il of tube 5 is connected to the negative terminal ofresistor 'i through a path including the coil of the input circuit il; acondenser l2 of low impedance to carrier and audio frequency currentsbeing connected in shunt across that portion l of resistor 'I which isconnected between the cathode and the grid of tube 5. The condenser i3connects the cathode tap on resistor l to ground for I. F. currents.

` The audio voltage developed across resistor 9 is transmitted throughthe condenser-resistance coupling lll to the following audio ampliiierand it will be understood that one or more amplifiers may follow thelatter; the amplified audio energy will then be reproduced in anydesired type of reproducer. The condenser I5, shunted across loadresistor 9, bypasses carrier frequency currents around the resistor. Thenormal, or maximum amplication, bias for the input grid of amplifier Iis secured by maintaining the cathode of tube I at ground potential, andconnecting the low potential end of input circuit 2 to the'negative endof load resistor 9. A degenerative path is provided which comprises alead I6 which has in circuit therewith a variable resistor I'I. The gridend of adjustable resistor I'I is connected to ground through a networkwhich includes variable resistor I8, coil I9 and condenser 20 in series,the variable condenser 2l being connected in shunt with the last namedthree series elements.

The potential across the grid bias resistor section 'I' is such as tobias the grid II for normal plate rectification by tube 5. As is well.known, the detection characteristic of such a biased detector hasconsiderable curvature in the weak signal input region, andapproximating linearity for strong signal inputs. The non-linearity ofthe characteristic introduces distortion into the audio output. Byfeeding back signal voltage, developed across coil I il in degenerativephase, Ithe distortion introduced at the detector is cancelled out. Itwill be observed that the variable resistor Il functions to controlvolume; it can act as the manual volume control device of the receiver.The normal voltage (D. C.) across resistor 9 maintains the grid of tubeI at a normal negative bias with respect to itsv grounded cathode. Asthe carrier amplitude at the signal collector increases, the D. C.voltage across resistor 9 increases with the result that the gain ofampliier I is decreased. Prior tubes may be gaincontrolled, in additionto tube I, by tapping olf the D. C. voltage component from lead I 6 andtransmitting the voltage to the controlled tubes through an audio lter22. Either of the elements I8 or 2| may be used as a tone controlelement. Thus, increasing 2I will increase the high audio frequencycomponents; increasing resistor I8 will decrease low audio frequencies.The condenser 20 grounds the grid of tube I for I. F. currents, butblocks direct current.

In Fig. 2 is shown a modication wherein the biased detector tube feedsaudio voltage, developed across load resistor 9, in degenerative phaseto the detector input circuit LI. Feedback is accomplished by connectingthe path including variable resistor Si) and condenser 3i between theplate end of resistor 9 and the low potential end of the detector inputcoil. The AVC bias is transmitted 'over the lead 32 to the grids of thecontrolled signal transmission tubes. The AVC path includes anappropriate audio filter network 33. Proper energizing potentials forthe amplifier and detector tubes is provided by the common voltagesupply potentiometer lil). The. plate 8 is connected to a desiredpositive voltage point lil; the cathode 6 of the detector tube isconnected to a relatively negative point 42. The cathode I of amplifierI is connected to point 43 which is negative with respect to point lil.The normal negative bias (maximum gain) of amplifier I is secured byhaving the normal voltage across resistor 9 exceed the potentialdifference between points 4I and 43 by the value of the desiredno-signal negative bias.

Grid II of the detector tube 5 is maintained at the normal negative biasbetween ground and point 42; the grid being connected to ground byresistor 59 which is shunted by the bypass condenser 5I. Network 5ft-5Ihas a low I. F. impedance but high audio impedance. It 1s across thisimpedance that the fed back audio current through {t-3l builds upvoltage to apply in inverse phase to grid II. Resistor 30 acts as amanual volume control device; it adjusts the degree of negative feedbackthrough path Sii-SI. The degenerative, or negative, audio feedback inthe detector stage acts to reduce the harmonic distortion due to thenon-linearity of the detection characteristic. Since the tube functionsas a biased detector, amplification of the audio voltage is secureddespite the reduction in audio intensity due to negative feedback Thecharacteristic of the detector is made to approach linearity.

Fig. 1 diifers from Fig. 2 in that feedback in Fig. 1 is of the I. F.components including the audio sidebands. The output of a detectorcontains not only audio frequency components, but also the radiofrequency components (I. F. and sidebands) present in its input circuit.Ordinarily these radio frequency components are bypassed, and the audiofrequency components only are utilized. In Fig. l the coil II) in theplate circuit has I. F. and sideband voltage built up across it; thisvoltage isv fed back through lead EG and feedback control impedance Ilto the circuit 2I-I8-IEi-2 which is likewise an I. F. impedance,although usually of lower magnitude of I. F. impedance than circuit 2.The I. F. component fed back is so phased that the original modulationcomponents are opposed. As is well known, distortion in a square law'detector is proportional to the square of the modulation; feeding backenergy to oppose the original modulation of .the I. F. wave decreasesdistortion not only by feedback action as in Fig. 2, but, also, byreduction of eifective input modulation.

While we have indicated and described several systems for carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means limi-ted to the particularorganizations shown and described, but that many modications may be madewithout departing from the scope of our invention, as set forth in theappended claims.

What we claim is:

l. In a detection network, a tube having at least a cathode, a plate anda control grid, a signal input circuit connected between the grid andcathode, a resistive impedance connected between said plate and cathode,a path of low impedance to audio frequency currents connected between apoint on said resistive impedance and said control grid, said path beingconstructed to impress audio voltage developed across said impedanceupon said grid in degenerative phase with the audio modulation on thecarrier at said input circuit, a second connection .to said impedancefor deriving therefrom solely a direct current voltage developedlthereacross by the tube space current, and means for biasing the grid sothat the network detects by vplate rectification.

2. In a detection network, a tube having at least a cathode, a plate anda control grid, a signal input circuit connected between the grid andcathode, a resistive impedance connected between said plate and cathode,a path of low impedance to audio frequency currents connected between apoint on said resistive impedance and said control grid, said path beingconstructed to impress audio voltage developed across said impedanceupon said grid in degenerative phase with the audio modulation on thecarrier at said input circuit, and means for biasing the grid so thatthe network detects by plate rectification, an amplifier tube precedingsaid detector tube, and direct current voltage connections between theamplifier input electrodes and said impedance whereby the detector spacecurrent flow through said resistive impedance provides bias for saidamplifier tube.

3. In a detection network, a tube having at least a cathode, a plate anda control grid, a signal input circuit connected between the grid andcathode, a resistive impedance connected between said plate and cathode,a path of low impedance to audio frequency currents connected between apoint on said resistive impedance and said control grid, said path beingconstructed to impress audio voltage developed across said ime pedanceupon said grid in degenerative phase with the audio modulation on thecarrier at said input circuit, and means for biasing the grid so thatthe network detects by plate rectification, said path including anadjustable resistor .to vary the feedback therethrough over a wide rangeof values whereby the adjustable resistor acts as a manual volumecontrol device.

4. In combination with a biased detector tube having a signal inputcircuit and an audio output circuit, means, including a series path ofcapacity and variable resistance, for degeneratively feeding audioenergy from the output circuit to said input circuit, a signal amplierfeeding signals to said input circuit, and means, responsive to thevariation in intensity of the direct current voltage in said outputcircuit, for varying the amplifier gain.

DUDLEY E. FOSTER. GARRARD MOUNTJOY.

